1. Field of the Invention
The present invention relates to dispersion strengthened aluminum-base alloys, and more particularly to a method for tungsten inert gas welding of these alloys.
2. Description of the Prior Art
In recent years the aerospace industry has searched for high temperature aluminum alloys to replace titanium and existing aluminum alloys in applications requiring operating temperatures approaching 350.degree. C. While high strength at ambient and elevated temperatures is a primary requirement, certain design applications mandate that candidate alloys also exhibit, in combination, ductility, toughness, fatigue and corrosion resistance, as well as lower density than the materials currently being used.
To date, the majority of aluminum base alloys being considered for elevated temperature applications are produced by rapid solidification. Such processes typically produce homogeneous materials, and permit control of chemical composition by providing for incorporation of strengthening dispersoids into the alloy at sizes and volume fractions unattainable by conventional ingot metallurgy. Processes for producing chemical compositions of aluminum base alloys for elevated temperature applications have been described in U.S. Pat. No. 2,963,780 to Lyle et al., U.S. Pat. No. 2,967,351 to Roberts et al., U.S. Pat. No. 3,462,248 to Roberts et al., U.S. Pat. No. 4,379,719 to Hildeman et al., U.S. Pat. No. 4,347,076 to Ray et al., U.S. Pat. No. 4,647,321 and U.S. Pat. No. 4,878,967 to Adam et al. and U.S. Pat. No. 4,729,790 to Skinner et al. The alloys taught by Lyle et al., Roberts et al. and Hildeman et al. were produced by atomizing liquid metals into finely divided droplets by high velocity gas streams. The droplets were cooled by convective cooling at a rate of approximately 10.sup.4 .degree. C./sec. Alternatively, the alloys taught by Adam et al., Ray et al. and Skinner et al. were produced by ejecting and solidifying a liquid metal stream onto a rapidly moving substrate. The produced ribbon is cooled by conductive cooling at rates in the range of 10.sup.5 .degree. to 10.sup.7 .degree. C./sec. In general, the cooling rates achievable by both atomization and melt spinning greatly reduce the size of intermetallic dispersoids formed during the solidification. Furthermore, engineering alloys containing substantially higher quantities of transition elements are able to be produced by rapid solidification with mechanical properties superior to those previously produced by conventional solidification processes.
Conversion of rapidly solidified aluminum base alloys disclosed in the aforementioned inventions and particularly those taught in U.S. Pat. No. 4,878,967 and U.S. Pat. No. 4,729,790 are accomplished by the processes disclosed in U.S. Pat. No. 4,869,751, U.S. Pat. No. 4,898,612. In U.S. Pat. No. 4,869,751 to Zedalis et al. there is disclosed a dispersion strengthened, non-heat treatable aluminum base alloy formed into useful shapes that include extrusions, forgings and sheet. U.S. Pat. No. 4,898,612 to Gilman et al. discloses use of a friction actuated process to fabricate extrusions directly from rapidly solidified aluminum base alloy powder.
One of the major restrictions to the widespread utilization of high temperature aluminum alloys is their inability to be joined using welding or brazing technologies. The application of conventional welding and brazing practices to conventionally processed high performance aluminum alloys results in the formation of excessive porosity in the weld and heat affected zone of the joint due to the outgassing of the alloy during the joining cycle and the coalescence of the gases to form porosity. The excessive gas porosity is caused in part by the presence of hydrogen, as hydrate, hydroxide or water, in the base metal. Also, the slow cooling of the welded area may favor the formation of coarse, brittle intermetallics which will severely reduce the joint strength and ductility when compared to the base metal. Finally, any treatment given to these alloys to improve their weldability must be cost effective.
The hydrogen content may be reduced by heat treatment of the high temperature aluminum alloy in vacuum at high temperature. However, excessive heat treatment causes a reduction of the base metal strength. Previous disclosures have shown that the weld porosity in powder metallurgy aluminum alloys (Al-10Fe-5Ce) can be virtually eliminated by a combination of preweld vacuum heat treatment, i.e. 750.degree. F. for 24 hrs. in vacuum, and direct current electrode negative welding, with only a minor decrease in base metal tensile strength. However, the welds exhibit a brittle behavior due to brittle phases formed near the weld interface. These welds are restricted to non-structural applications. (Gas Tungsten Arc Welding of Al-10Fe-5Ce, Guinn Metzger, report No. AFWAL-TR-87-4037, AFWAL/MLLS, Wright-Patterson AFB, OH 45433, February 1987).
U.S. Pat. application of Gilman, Ser. No. 650,122, filed Feb. 4, 1991 now pending, discloses a process for reducing the gas levels of rapidly solidified aluminum alloys by subjecting a compacted billet having a density varying from 70% to 98% of full density to a vacuum autoclaving treatment at 350.degree. C. to the incipient melting point of the alloy. While these techniques have been successful in eliminating the porosity generated from the welding process, the weldments are still subject to the formation of the coarse brittle intermetallics which will severely reduce the joint strength and ductility when compared to the base metal. Also, the high temperature properties of these alloys may be compromised by the formation of the coarse intermetallics.
Considering the dependency of their superior mechanical properties on the special microstructures of these rapidly solidified, dispersion strengthened alloys, it is apparent that joining techniques which can recreate and/or retain the unique microstuctural characteristics of the base metal need to be utilized in order to achieve high joint efficiency. Current references sight that among the fusion welding processes currently available to join aluminum alloys, high energy density processes such as electron beam and laser welding offer the greatest potential for achieving the recreation and/or retention of the rapidly solidified microstructure.
Gas tungsten-are welding (TIG welding) is an arc welding process in which the heat is produced between a nonconsuming electrode and the work metal. The electrode, the weld puddle, the arc, and adjacent heated areas of the work piece are protected from atmospheric contamination by a gaseous shield. Typically the high energy densities and low total energy inputs needed for the welding of rapidly solidified, dispersion strengthened aluminum are not attainable through TIG welding.
The need remains in the art for a process for welding rapidly solidified, dispersion strengthened aluminum base alloys while retaining useful mechanical properties.